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How to measure transformer oil temperature?

Transformer is an important primary equipment in the power system, and its operational reliability has a great relationship with the safe and reliable operation of the power system. According to relevant statistics, the average accident rate of transformers of 110kV and above is about 0.69%, among which, due to the over-temperature operation of the winding, the insulation is aged, the winding is broken down, and the transformer burnout accident accounts for a considerable proportion. The heavy gas protection in the non-electrical protection of the transformer actually indirectly reflects the abnormal temperature of the transformer.

It can be seen that the measurement of transformer temperature is of great significance for the early warning and timely action of transformer accidents.


The service life of a transformer depends on its winding temperature, which plays a decisive role in the insulating material. When the insulation temperature of the transformer winding is in the range of 80 to 130 °C, the insulation aging rate will double for every 6 °C increase in temperature, that is, the insulation life will be reduced by 1/2, which is the "six degrees rule" of insulation aging.

 

 

 

During the operation of the transformer, it is generally stipulated that 85°C is the limit of the upper oil temperature. In Dongying area, when the upper oil temperature reaches above 80°C, the alarm signal "XX main transformer body oil temperature high alarm" and "XX main transformer body oil temperature" will be issued. high warning".

 

The operating temperature of oil-immersed transformers, including transformer oil temperature and transformer winding temperature. The transformer is generally equipped with a temperature measuring tank inserted into the transformer oil on the top of the casing, and the temperature of the transformer oil is measured by setting a temperature measuring element in it. Since the object of measurement is the oil on the top layer of the transformer, we usually call it the oil temperature of the top layer of the transformer. Since heat is generated when the transformer windings are overloaded, the temperature of the transformer windings in operation must be higher than the temperature of the transformer oil. Among them, the most serious influence on the aging of the transformer insulation is the temperature of the hottest spot inside the transformer, which we call the transformer hot spot temperature. A transformer hot spot is typically somewhere near the ends of the transformer windings. Due to the inability to accurately locate and measure, we often use the transformer winding temperature to replace the transformer hot spot temperature as a means to monitor the internal hot spot temperature of the transformer. Therefore, oil-immersed transformers are generally equipped with a transformer top oil temperature gauge and a transformer winding temperature gauge.

The black pointer on the thermometer indicates the actual operating temperature, and the red pointer indicates the set upper limit alarm temperature. When the oil temperature of the upper layer of the transformer exceeds this value, it will alarm, the two pointers collide to make the electrical contact conduct, and an alarm signal will be issued, and there is a protrusion on the red needle. When the oil temperature exceeds the maximum temperature during operation, the black pointer will drive the red needle. The pointer rotates, at this time the red needle indicates the highest temperature reached during operation

 

The transformer winding temperature can be considered as the superposition of the oil temperature of the top layer of the transformer and the temperature rise of the winding to the oil. Generally, the load current of the transformer is added on the basis of the transformer oil temperature measurement system, and the temperature rise of the winding to the oil is simulated by the thermal simulation method, so as to finally obtain the transformer winding temperature. The schematic diagram of the specific implementation method is shown in the following figure.


Oil-immersed transformer winding temperature Tw can be equivalent to:

 

Tw=T0+k*△Two

  

In the formula, T0 is the oil temperature of the top layer of the transformer, △Two is the copper oil temperature difference, obtained by thermal simulation, k is the thermal coefficient, which is related to the transformer capacity, winding structure, etc. The temperature rise of the winding to the oil △Two depends on the winding flow. passing current.

 

 

 

The measurement system is mainly composed of a temperature sensing package, a temperature measurement compensation circuit, and a sensor. Among them, the temperature measurement compensation circuit and the temperature sensor are all in the temperature sensing package, and the temperature sensing package is immersed in the top oil of the transformer to sense the top oil temperature. The sensor consists of two parts: the temperature sensor connected to the on-site winding temperature mechanical meter, the collected temperature value is reflected in the temperature indicator plate reading; the Pt100 thermal resistance, whose resistance value changes with the temperature change, the collected temperature value is used to send the computer Background monitoring system.

 

 

 

The oil temperature information is transmitted to the dispatch center through the connection method shown in the figure below to realize real-time monitoring of the upper oil temperature and winding temperature.



1. How to judge whether the temperature change of the transformer is normal or abnormal?

 

 

 

During the operation of the transformer, the losses in the iron core and windings are converted into heat energy, which causes heat in various parts, which increases the temperature, and the heat is diffused to the surroundings by means of radiation and conduction. When the heat generation and heat dissipation reach a balance state, the temperature of each part tends to For stability, the iron loss is basically constant, while the copper loss varies with the load. When inspecting the transformer, record the external temperature, top oil temperature, load and oil level height, and compare and analyze with the previous values to judge whether the transformer is operating normally.

 

 

 

If it is found that the oil temperature is more than 10°C higher than usual under the same conditions, or the load remains unchanged but the temperature continues to rise, and the cooling device operates normally, it is considered that the transformer has an internal fault (pay attention to whether the thermometer has errors or failures), generally The main insulation of the transformer (winding insulation) is Class A insulation (paper insulation), and the maximum operating temperature is 105°C. Generally, the winding temperature is 10~15°C higher than the oil surface temperature. If the oil surface temperature is 85°C, the winding temperature will reach 95~100°C.

 

 

 

 

2. Reasons for abnormal temperature of transformer

 

 

 

① Abnormal temperature caused by internal fault

 

 

 

Transformer internal faults such as inter-turn short circuit or inter-layer short circuit, the coil discharges to the enclosure, the internal lead joints are heated, the multi-point grounding of the iron core increases the eddy current and overheats, and the leakage flux such as zero-sequence unbalanced current forms a loop with the iron oil tank and heats up When the temperature of the transformer is abnormal due to other factors, it will also be accompanied by gas or differential protection action. When the fault is serious, the explosion-proof pipe or the pressure relief valve may be injected with oil. At this time, the transformer should be stopped for inspection.

 

 

 

②The abnormal temperature caused by the abnormal operation of the cooler

 

 

 

Abnormal temperature caused by abnormal operation or failure of the cooler, such as outage of submersible pump, damaged fan, fouling of cooling pipes, poor cooling efficiency, and failure of radiator valve to open. The cooling system should be maintained and flushed in time or a backup cooler should be put in, otherwise the load of the transformer should be adjusted.

 

 

 

③ If the temperature indicator has errors or the indication fails, the thermometer should be replaced.


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