Early Warning and Monitoring System for Power Cable Joint Faults in Power Automation Equipment Wang Xinchao, Pan Yicun (School of Electrical Engineering, Shandong University, Jinan 250061, China) An indirect online evaluation of joint resistance and aging algorithm, theoretically verified the joint temperature and joint quality Sex. Based on this, the cable joint temperature monitoring system based on the above mechanism is introduced. The effectiveness and credibility of the mechanism are illustrated by an example analysis, and the advancement and reliability of the system are also verified.
In the power supply systems of cities and large and medium-sized enterprises, power cable transmission and distribution is increasingly used. When the power supply distance is long, cable joints usually appear on the line. Years of operation experience show that more than 90% of cable operation failures are caused by joint failures. Further analysis shows that contact resistance, overload and other factors are the main causes of excessive joint temperature, resulting in insulation aging or collapse failure at the cable joint. To this end, a temperature-collecting cable joint temperature monitoring system capable of simultaneously monitoring temperature changes of multiple cable joints, real-time display, timed printing and recording of joint temperature, and comprehensive analysis and alarm function has been developed, so that the operator can understand the cable in time and accurately. The operation situation effectively avoids the occurrence of hidden dangers and ensures the safety of power transmission.
1 Theoretical basis and algorithm for temperature monitoring Innumerable times, the joint temperature is the main parameter reflecting the joint quality. When the contact resistance of the joint increases, the joint temperature will also increase. The high temperature for a long time will lead to insulation degradation at the joint. If the joint temperature is continuously tested and optimized, the joint quality changes can be analyzed and grasped in time to identify possible faults. In order to clarify the relationship between the resistance of the joint and the temperature, the following analysis was made.
1.1 Joint heat source joint heating is mainly the heat energy released by the loss of current flowing through the line at the joint resistance. The heat energy is proportional to the square of the current flowing through the line and the resistance value of the joint.
1.2 The heat transfer heat is mainly transmitted in three directions, that is, the two sides of the guide wire are transmitted and transmitted outward through the joint insulation. Due to the existence of thermal resistance, the temperature is distributed along the wire according to a certain gradient. The temperature at the joint is the highest, gradually decaying to a certain distance to reach the ambient temperature; the heat transmitted along the joint insulation, the thermal resistance through the insulation and a certain range of air thermal resistance Transition to ambient temperature.
1.3 The electric and thermal hybrid network diagram indicates that Ti represents the temperature at the bare joint, T is the cable trench ambient temperature, TX is the measuring point temperature, Rx is the contact resistance at the joint, Ix is the cable current measured by the current transformer, and Rh is the cable. The two ends are converted to the equivalent thermal resistance at the joint, Rm is the equivalent thermal resistance of the joint to the temperature detecting unit, and RH2 is the equivalent thermal resistance of the temperature detecting unit to the ambient temperature. Due to the high voltage of the cable, from the perspective of safety, ease of installation and cost, it is necessary to obtain T. The cost is relatively high. Therefore, the indirect temperature Tx is used in the subsequent analysis. The selection of components and materials is the heat capacity of the temperature measuring component. To be small, a material with high insulation and heat conductivity should be injected between the measuring point and the joint, and a heat insulating material should be used between the measuring point and the environment. After the above treatment, the calculation result of replacing T with Tx actually proves that the requirements of engineering calculation can be fully satisfied.
1.4 Algorithm and formula Based on the hybrid network and related theorems, the relationship between current, temperature and resistance can be derived as follows: Example correction factor.
The edge material (high-order polymer) is very easy to react with oxygen, etc., resulting in a general use of the case when subjected to temperature and light, external force can be achieved. Its 6-way scrolling WEDc display bookmark5 is visible from the above formula. When the electrical equipment is running normally (its load current does not change greatly), the joint resistance is proportional to the temperature difference (Tx-T), that is, the cable head can be used. The temperature is used as an early warning signal for cable head failure.
In addition to the extreme warning, it is necessary to take measures on the cable head in time. It is more meaningful to evaluate the quality of the non-warning cable head during the overhaul of the equipment and find out the hidden dangers. Because the repairs at this time will not affect the production.
This kind of evaluation mainly starts from whether the joint resistance has a qualitative change, the joint insulation aging degree and so on. Due to the aging of the joint insulation, the insulation strength and the sealing property are degraded, which brings potential potential hazards such as leakage and accelerated joint oxidation, which is of practical value. The following is a discussion of the basis and method of evaluation for these two questions.
In the formula (1), since the initial value parameter is not easy to obtain (most of the temperature probes are added after the line operation, that is, the technical improvement of the existing equipment), direct use is inconvenient. However, in engineering practice, it has been found that it is possible to identify hidden dangers by monitoring whether the joint resistance has changed significantly in the recent period of time. Assume that the joint performance is good before ti time (generally the temperature probe can be taken), and the ratio of the joint change can be obtained as a standard compared with the current joint performance at a certain time t2 (this ratio can be used as a A detection device works independently, and its alarm temperature setting, print setting, and acquisition path setting can all browse the connected measuring point signals. This independent design is to reflect the characteristics of the distributed system and improve the reliability of the whole system. However, such on-site workstations do not have evaluation software and graphics display capabilities, and their history records only alarm points. These optimization functions are implemented by the host computer.
The relay unit mainly has two functions: a. Querying each field collection processing workstation by address number downwards or relaying instructions to superiors, retrieving information and caching and forwarding upwards; b obeying the setting and management of the upper computer Subordinate networks, these settings include the number of stations accessed, address number, information content, transmission rate, timing interval, and so on.
Both the attendant observatory and the engineering station have query and print functions, but the work permissions are different. In addition to the functions of the usual attendant observation station, the engineering station also has comprehensive functions such as alarm temperature setting, internal network configuration setting, and acquisition data management. The entire system can be connected to the internal LAN of the enterprise through network cards and hubs to obtain relevant data support.
3 application examples analysis 3.1 temperature alarm example fault phase A fault line number: heavy oil hydrogenation A 8 3 (connector 3) alarm temperature: 65C history as shown in (a). (May-July 998).
From the load current, the current load is too high 2 months before the alarm. After checking the maintenance record, it is the capacity of the No. 9 line to replace the cable, and part of the load is incorporated into the A8 line. Fault conclusion: the original joint is not well sealed, the water in the cable trench is wet, the joint is corroded, and the load is subjected to a load shock, resulting in an increase in joint resistance. Remedy: Re-create the joint.
In this example, the temperature difference (Tx-T) curve at the cable head basically reflects the change in the joint temperature. Although the curve is affected by the load current, it is still possible as a fault alarm signal. For the field workstation (lower position machine) with relatively low environmental configuration and high reliability requirements, the temperature difference (Tx-T) curve is used as the basis for evaluation, which is more convenient to use.
3.2 Insulation update evaluation example Fault line number: Heavy oil hydrogenation A 6 (1 connector) evaluation recommended insulation update phase B; recommended level: three levels; history as shown in the load current, about one and a half years before maintenance The load current is large, and the temperature stays at 57*C for a long time. Theoretical analysis may be that the original resistance of the joint itself is high (although the resistance evaluation does not give much change). Field investigations revealed that there were small cracks on the surface of the insulation layer, and there were small holes in the outer layer and a pungent gas was released. Conclusion: The quality of the original joint is not high, the joint temperature is high, and the insulation layer is aging ahead of time. Treatment: Repair the joint and insulation.
4 Conclusion The system has complete data acquisition, transmission, processing, display, printing and long-distance communication capabilities, and is equipped with powerful software support. It can be seen from the design structure that it can be used not only for the performance of cable joints. Dynamic monitoring can also be combined with other detection components to achieve real-time monitoring of various industrial control quantities. If gas sensors are used, cable trenches or other transport conduits such as hazardous gases are used to monitor harmful gas concentrations.
The system has been successfully put into trial operation of heavy oil hydrogenation opening and closing in Qilu Petrochemical Shengli Refinery. For more than 4 years, the device has been stable in performance and good in operation. It has successfully detected faults in 3 times, reduced direct losses by more than one million yuan, and indirectly lost millions of dollars, which has brought huge benefits to enterprises. The safety of the power grid and production in the plant area has a good promotion value.
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