Text Box: Impulse Frequency Response Analyzer MRF-100i

Overview

MRF-100i is a portable testing equipment intended to determine the frequency characteristics
(transfer function) of the power transformers, instrument transformers, reactors, etc.

The frequency characteristics is analysed to identify the winding deformations and displacements caused by:

(i) the electrodynamic forces generated by the fault currents,

(ii) mechanical stresses appearing during transportation and in operation.

    Transformer under test
The test equipment MRF-100i

MRF-100i consists of an impulse generator, a digital acquisition unit and a laptop running an application software.

The frequency response is determined applying low voltage impulses, having a frequency spectrum as broad as possible. The digital acquisition unit synchronously records the applied signal and the response signal. The two signals are software processed to determine the transfer function.

The excitation signal produced by the impulse generator has an amplitude of 600 V maximum and the shape of a chopped lightning impulse with a chopping time Tc of about 0.8 ms.

The measured signals are taken over by means of a voltage divider and a shunt, directly mounted at transformer terminals.

All necessary testing cables, together with the terminal fixing clamps, the voltage divider and the shunt are included in the standard configuration of MRF-100i. 

Theory  

The winding of every transformer has associated an unique RLC network depending on its geometry, used materials and manufacturing. Following a mechanical stress a change of the RLC network appears. It means a change of the frequency response characteristics that can be identified by a comparative analysis with a reference case (at factory gate and/or after mounting in the transformer station).

Transformer state check by frequency response analysis is made comparing the present characteristics with the previously determined ones for each phase. The pole number variation or resonance frequency displacement means a change of the distributed winding capacities and inductivities, namely a change of the mechanical state.

The transfer function is calculated using the spectral density estimators, with a view to reducing the measuring noise. The frequency ranges where the noise is high are indicated by coherence function values. The coherence function has amplitudes from one to zero. The value "1" indicates a correct measurement with a negligible noise and the value "0" indicates an incorrect measurement (non-linear relation between the applied signal and the response signal).

        

 MRF-100i Main Technical Characteristics

 

   
Frequency characteristics measured on the HV winding of a autotransformer 200 MVA, 231/121/10,5
Frequency characteristics measured on the HV winding of a transformer (45 MVA, 132/33 kV)

MRF-100i testing equipment was designed and achieved especially for on-site measurements. The easiness of its mounting on the transformer, the low weight and small number of components enable the quick transport and installation where it is used.