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Characterization in the frequency domain


 Frequency characterization consists of non-destructive and non-invasive tests applicable to the monitoring and diagnosis of power electrical equipment.


This technique has been widely used, in particular for power transformers and reactors. The focus of the use of frequency characterization can be summarized as follows:


• evaluation of the integrity of the equipment throughout its life cycle;

• evaluation during type tests;

• identification of natural resonances and anti-resonances of the equipment;

• aid in the modeling of equipment for electromagnetic transient studies;

• evaluation of geometric displacements of the windings.


Frequency characterization in power electrical equipment is obtained from the measurement of the terminal impedance and the frequency response between the terminals, usually in the frequency range 10Hz to 2MHz.Measurement of the terminal impedance of the equipment at various frequencies thus characterizes the terminal impedance behavior over a range of frequency. The frequency response is based on the measurement of the voltage transferred at frequencies other than 60 Hz, the output signal and the input signal of each side of the equipment being related in a specified frequency range.


The measurements are somewhat simple, however in the case of a power transformer, the winding configurations as well as the type of transformer will determine some parameters, such as the types of connections, number of measurements and amplitudes of the signals to be monitored.Because they are "off-line" techniques, the terminal impedance measurement as well as the frequency response must be performed with the transformer fully disconnected from the system.

Cepel has been working in the area of diagnostics in electrical equipment for many years using this technique in power transformers, new and in operation, of several Brazilian utilities. The experience in testing power transformers is based on more than one hundred equipment evaluated. In relation to the characterization in the frequency domain, the main activities developed are:


• research and development of methodologies for the diagnosis of high voltage equipment
• search and classification of patterns and application of artificial intelligence
• formation of database of signatures in frequency response and terminal impedance
• providing technical support to implement the techniques in electric energy companies
• personal training
• participation in standardization committees
• participation in national and international congresses and seminars;
• more than one hundred transformers and reactors evaluated by the technique, on site and in the laboratory



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