Transformer core multi-point earth-fault approach
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Abstract: The core transformer for one reason or another for more than two take place, it will form a closed loop, resulting in circulation, which is the core of the so-called multi-point ground fault. It can cause a short circuit core local overheating or even burning core local and other major failures.
Key words: short circuit; circulation; insulation resistance; grounding
Key words: TM40 Document code: B
 
At present, China-made large and medium-sized core of the transformer are cited as a casing to the external fuel tank-grounding. In the normal operation of the power transformer, the existence of the electric field around the winding, and core and folders, and other pieces of metal components in the field are. If the core is not a reliable ground, then have a charge and discharge, damage to the insulation. Therefore, the core must have a reliable grounding point. If the core for one reason or another then another place to form a closed loop, the normal grounding of the lead will be on circulation, on the one hand, local short-circuit caused by overheating core, serious, local burning core; On the other hand, As the core of the grounding line have a normal circulation, causing local overheating transformer may have a discharge of failure. Here transformer core multi-point analysis of the ground and Service
Li methods.
     First, iron-chi points to judge the earth fault
     1. Cores measurement of insulation resistance
     If the insulation resistance is zero or very low, it may indicate the existence of core earth fault.
     2. Ground surveillance line in circulation
     Folder on the core or small pieces through the casing caused by the grounding transformers, should routes in the ground to monitor whether there is a general circulation, and if so, should make the transformer outage, the core of the insulation resistance measurement.
     3. Gas chromatography analysis
     Of oil gas for gas chromatography analysis, also found that the most effective grounding transformer core. Emerging core of the earth fault transformers, the oil chromatography analysis of the data, the total hydrocarbon content over the "transformer oil dissolved gas and guidelines to judge" (GB7252-87) provides that the value of attention (150μL / L), one of ethylene (C2H4 ), Methane (C2H2) low-or no, that does not meet the requirement if people pay attention to the value of more than attention to the value of acetylene (5μL / L), may be dynamic earth fault. Gas chromatography analysis can be integrated with the two methods used to determine whether the core multi-point ground.
     Second, simple on-site treatment
1. Not hanging core of the provisional series limiting resistance
Found that more core transformer ground fault, the power to be suspended for inspection and processing core. The temporary system does not allow inspection of the power failure, can be cited out on the road back to the core grounding resistance then apply the temporary emergency measures to limit the increase in circulation to prevent further deterioration of the fault. In the series resistance before the respective cores measurement of ground loop of circulation and open circuit voltage, and then calculate the resistor string resistance. The string should not be too much resistance in order to protect the fundamental core to a potential; should not be too small to limit the circulation in the following 0.1A. At the same time need to pay attention to the series resistance of the heat capacity to prevent resistance caused by burn core open.
     2. Hanging core inspection
(1) measurement division of the folder or Chuanxin screw on the core (two and a half-core will be merging to open in the middle) of the insulation in order to gradually narrow the scope of the failure to find.
     (2) Check the gap, the Department of tank focused on whether the metal parts of inclusion.
     (3) remove or core insulation pads on the sludge or rust on the bottom of the core can not see a place with clean-up wire.
     (4) of the gap or nitrogen washing oil washed clean blow.
     (5) with a hammer percussion pieces of vibration folder at the same time by monitoring the shake table to see whether changes in the insulation, then find and eliminate the dynamic place.
     3. The impact of the discharge law
     As the transformer body in the air should not be too long exposure time, as well as the transformer assembly in the form of constraints, the scene many cases, unable to find the exact location of access, especially welding slag suspended due to rust, caused by oil sludge deposited more ground, it is difficult to find. The impact of such failure may discharge method, an approach that depends on the specific circumstances at the scene, grounding and the extent of Grounding, hanging in the core or non-core suspended state can be carried out.
     On-site methods used mainly in capacitance-voltage direct current law, the exchange of electric welding current law and the impact of the discharge law. AC electric welding current law applies only to metal and earth fault, but the current poor control, and this situation would be very rare at the scene, most of the grounding resistance in more than a few hundred ohms. Capacitance-voltage direct current method based on the scene more difficult to operate without the inconvenience security, we should not promote. According to the author's experience, the impact of the discharge is a safe, simple, rapid and beneficial use of the materials available. It uses high-voltage electrical tests carried out by a step-up transformer, a good conversion experiments attention to the secondary voltage, as the core of the insulation pads and thin, it can not be higher than the second voltage 2.5KV.
     Third, the proposed
     1. In the operation of the transformer core grounding in the best online ammeter to install, easy to find fault in a timely manner. In particular, the impact of the discharge method to eliminate the phenomenon of grounding, but also stepped up surveillance to prevent the formation of failure again.
     2. When the multi-core earth fault, in an integrated and comprehensive analysis of inspection measured, as the case may choose to deal with the program, must not blindly carry out the impact of discharge or burning in addition to welding, so as not to cause damage to the insulation, the expansion of failure.
     3. Every time hanging overhaul core, we must clean sludge at the bottom of the fuel tank debris, such as rust, oil and a comprehensive irrigation.
     4. Submersible pump to enhance the maintenance and cooler, prevent or metal bearings wear off, resulting in multi-core transformer grounding fault.