变压器油是一种电介质，是能够耐受电应力的绝缘体。当对介质油施加交流电压时，所通过的电流与其两端的电压相位差并不是90度角，而是比90度角要小一个δ角的，此δ角称为油的介质损耗角。变压器油的介质损耗因数 用介质损耗角δ正切值（tgδ）来表示。
Transformer oil is a kind of dielectric, which can withstand electrical stress. When the AC voltage is applied to the dielectric oil, the phase difference between the current passing through and the voltage at both ends is not 90 degrees, but a delta angle smaller than 90 degrees, which is called the dielectric loss angle of the oil. The dielectric loss factor of transformer oil is expressed by the tangent value of dielectric loss angle δ (TG δ).
其中：
Among them:
Ig：充电电流，取决于电容，是无功电流，不造成任何损耗；
Ig: charging current, depending on the capacitance, is reactive current and does not cause any loss;
Ir：传导电流，为有耗电流，它造成所谓离子传导损耗，这种电流在直流电压和交流电压时都是一样的；
IR: conduction current, which is a lossy current, causes the so-called ion conduction loss. This kind of current is the same at DC voltage and AC voltage;
Ij：吸收电流，仅发生于施加交流电压时，它是由极化和偶极的转换所引起的，此种电流造成所谓偶极损耗。
Ij: absorption current occurs only when AC voltage is applied. It is caused by polarization and dipole conversion. This current causes the so-called dipole loss.
介质损耗因数是评定变压器油电气性能一项重要指标，特别是油品劣化或被污染对介质损耗因数影响更为明显。在新油中极性物质较少，所以介质损耗因数一般较小，如我国标准GB 2536-90要求企业在变压器油出厂时控制介质损耗数不大于0.005 ( 90℃)。使用一段时间后油的介质损耗因数过大就要采取处理措施，如采用真空过滤或用硅胶吸附处理等措施去除混入油中水分、微生物、氧化产物、外界颗粒物以及溶入油里的固体绝缘材料中的极性物质等。
Dielectric loss factor is an important index to evaluate the electrical performance of transformer oil, especially when the oil is deteriorated or polluted. There are few polar substances in the new oil, so the dielectric loss factor is generally small. For example, the Chinese standard GB 2536-90 requires the enterprise to control the dielectric loss no more than 0.005 (90 ℃) when the transformer oil leaves the factory. After using for a period of time, if the dielectric loss factor of oil is too large, treatment measures should be taken, such as vacuum filtration or silica gel adsorption to remove water, microorganisms, oxidation products, external particles and polar substances in solid insulating materials dissolved in oil.
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