روش طیف سنجی فرکانسی از روشهای مدرن برای تشخیص رفتار اجسام و پدیده ها در حوزه گسترده ای از علم و تکنولوژی می باشد. اساس کار آن نیز برسی پاسخ فرکانسی سیستمها در شرایط تعریف شده آزمایشگاهی است. این روش در صنعت فشار قوی نیز کاربردهای فراوانی دارد. مثلاً با تعیین میزان تلفات عایقی مواد در فرکانسهای مختلف و مقایسه آن با مواد استاندارد یا شناخته شده می توان به نتایج قابل توجهی در مورد طول عمر عایق یا ضعف عایقی آن بدست آورد. در زیر برخی کاربردهای این روش در صنعت فشار قوی تشریح شده است.
Frequency domain spectroscopy
This technique is a generalisation of the capacitance and dissipation factor (tanδ) or power factor measurements usually done at or close to power frequency (50/60 Hz). The difference is that several measurements at different frequencies are performed instead of a single measurement at fixed frequency. The measurement principle can be described as follows: a digital signal processing unit generates a sinusoidal test signal with the desired frequency. This signal is amplified with an internal amplifier and then applied to the specimen. The voltage over and the current through the specimen are measured with high accuracy using a voltage divider and an electrometer. As a result, the complex impedance, Z, is obtained as a function of frequency including its values close to power frequency as well. The diagram of the measurement set-up is shown in figure below.
From the impedance, the relevant parameters such as dissipation factors and capacitances are calculated. Moreover, if the geometrical capacitance of test object is known, one can obtain [another often used parameter – complex permittivity ε(ω).
The small bandwidth makes this method relatively insensitive to interferences, thus allowing for low voltages to be used. Using an efficient filtering of their corruptive effects can additionally reduce the influence of mains frequency and its harmonics.
By measuring the capacitance and dielectric losses over a frequency spectrum (rather than at a fixed frequency), plentiful information about the status of the insulating material can be obtained. It is for example possible to distinguish between different components and also to identify ageing processes in the insulating material.
For many ageing processes in insulation materials, detection is not facilitated by diagnosis using high measuring voltages. Rather, it could even be a disadvantage. Studies of oil/paper systems, such as power transformers, bushings, mass impregnated high and medium voltage cables, have shown that this type of insulation behaves linearly, i.e. its frequency response is independent of the applied voltage. Therefore low voltage is often sufficient for measurements.
Examples of areas where frequency domain spectroscopy as diagnostic tool is used today are: power and instrument transformers, bushings, oil/paper, and XLPE cables.
Dielectric properties of mineral oils
Transformer oil is a non-polar liquid with a capacitance that changes very little with frequency. The relative permittivity, εγ
, of oil is usually in a range 2,1-2,3. The dielectric loss of oil is mainly governed by its DC conductivity that varies considerably with the quality of the oil. As a result, measured dissipation factor, as a function of frequency will slope downward in a straight line as shown in figure below.
Dielectric properties of cellulosic paper
Oil impregnated cellulosic paper, e.g. transformer pressboard and paper cable Kraft paper, has a more complex structure and it is characterised by frequency dependence in both capacitance and loss. The qualitative description of paper usually is given using its moisture content, which increases due to ageing and exposure to atmosphere. Changes in moisture content in paper are easy to detect using frequency domain spectroscopy as shown in figure below.