Author: Noyafa–Cable Tester Suppliers
With the rapid development of the power system, the transmission line has developed a cable-overhead line hybrid transmission line on the basis of the original cable and overhead transmission line, and its application has become more and more extensive. The ultra-high voltage cable overhead line hybrid line can cross large waterways and straits, and can directly supply power to the center of large cities and industrial areas. At the same time, the cable-airline hybrid line is also used in low-current power transmission systems where the neutral point is not directly grounded, such as railway signal power supply systems.
However, due to manufacturing flaws or over a period of time, the insulation level of the cable can drop, which can cause a ground fault in the cable, and similar faults can occur in overhead lines. When the transmission line fails, accurate fault location can reduce the burden of line inspection on the one hand, and on the other hand, it can speed up the restoration of power supply and reduce the economic loss caused by power outage. With the wide application of cable overhead lines, its accurate fault location has become more and more important.
1 Cable fault location method for transmission lines The main types of faults in transmission lines are: single-phase-to-ground fault, interphase short-circuit fault, two-phase short-circuit-to-ground fault, and three-phase short-circuit fault. Among them, the probability of single-phase grounding fault is the largest (about 80%). This article takes single-phase grounding fault as an example to illustrate. For a long time, transmission line fault location technology has received extensive attention, especially since the 1970s, with the widespread application of computers, the cable fault location algorithm based on microcomputers and microprocessors has become a popular choice for relay protection workers at home and abroad. One of the research hotspots.
2. Fault location method for hybrid cable-overhead line . The mode of the hybrid line of the cable overhead line is no longer the mode of the uniform transmission line for the impedance method; for the traveling wave method, there are problems such as the inconsistency of the wave speed caused by the large difference in the wave impedance. The following introduces the fault location method according to the characteristics of mixed lines.
2.2 Segment fault location method based on distributed parameter model In a simple two-terminal power system, when a ground fault occurs at one point, the fault network is decomposed into a normal network before the fault and an additional positive network after the fault by using the symmetrical component method and the principle of linear superposition. , negative and zero sequence networks. For three-phase symmetrical faults, there are no negative-sequence and zero-sequence networks; for asymmetrical non-earthed faults, zero-sequence networks do not exist. 2.3 The double-ended traveling wave ranging method based on the wave velocity normalization algorithm 2.3.1 The wave velocity normalization algorithm The key of the traveling wave ranging algorithm is the accurate identification of the traveling wave head and the determination of the traveling wave propagation speed.
The obvious discontinuity of the wave velocity in the hybrid line becomes the bottleneck in the application of this method in the system, and the wave velocity normalization algorithm can solve this problem well. Let the propagation speed of the traveling wave in the overhead line be v, and the propagation speed in the cable section be v', the cable length is converted based on the wave speed v of the overhead line, and the converted length of the cable of length l is vl/v', the traveling wave propagation speed in the line after the normalized wave speed is the wave speed v of the overhead line. By applying the equation to the line after the normalized wave velocity, the position of the fault point in the normalized line can be obtained, and then converted to the original actual line, which can eliminate the influence of the discontinuous wave velocity.
2.3.2 Algorithm based on wave velocity normalization Due to the frequent alternation of overhead lines and cables, the double-ended traveling wave ranging makes the refraction and reflection of traveling waves in the line very complicated. Theoretical analysis and a large number of simulation experiments show that it is difficult to identify the traveling wave head reflected by the fault point by using the single-ended ranging method without signal source for this kind of line, which greatly affects the ranging accuracy. 2.4 Single-ended traveling wave ranging method with the same voltage pulse applied to the three-phase bus 2.4.1 Basic principle When a single-phase ground fault occurs in the system, after the system reaches a steady state again, the same voltage pulse is simultaneously applied to the three-phase bus of the measuring end. The karrenbauer transform is carried out on the collected traveling waves, and the line mode traveling waves are used for ranging.
Since the three-phase line is basically symmetrical, when the same voltage pulse is applied to the bus terminal at the same time, it is equivalent to adding a pure zero-sequence component, that is, the ground mode component, to the line. Therefore, in the line mode circuit of the line, it is equivalent to no When the power supply is applied, the transient traveling wave caused by the simultaneous application of voltage pulses to the three phases propagates forward completely along the ground mode path. Each traveling wave on the three-phase line is in the same amplitude and phase before reaching the fault point, while the line mode traveling wave is always zero before the ground mode traveling wave reaches the fault point, so that the influence of discontinuous wave impedance can be eliminated, that is, the traveling wave in the ground mode can be eliminated. Before the wave reaches the fault point, no matter how many overhead lines and cable switching points pass through, the induced line mode traveling wave is always zero. 2.4.2 Equivalent Wave Velocity and Wave Velocity Normalization Algorithm Through the above strategy, the reflected wave head of the fault point can be correctly extracted, but the distance measurement cannot be completed. This is because another key factor of distance measurement is the wave velocity. There is not a single line that is frequently exchanged, and a corresponding strategy must be found to finally complete the ranging.
The following conclusions can be drawn from the physical process of traveling wave propagation in the line: after the same voltage pulse is applied to the three-phase line, the added voltage pulse propagates along the line to the opposite end in the form of a wave. The hybrid line of cable overhead line is a relatively novel research object of cable fault location. Due to the large gap between the parameters of the cable line and the parameters of the overhead line, many existing fault location methods based on line uniform parameters have been challenged. . This paper reviews several existing hybrid line fault location methods, analyzes and compares the advantages and disadvantages of various methods, and draws the following conclusions: For cable fault location of cable overhead line hybrid lines, based on the wave velocity regression The double-ended traveling wave ranging method with one method has high feasibility and should be further promoted and used.
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