The authors of this paper proposed an effective method applied to frequency response interpretation in low frequency range and mechanical failure diagnosis support based on measurements and analysis of a physical lumped component transformer circuit. To extend the interpretation until medium frequencies where mechanical failures influence most, this paper presents a novel approach on determining parameters in a physical distributed component circuit. The most important contribution of this paper is the possibility of determination of electrical parameters in the distributed circuit without knowing detailed knowledge of the transformer construction, which has been a challenge so far. In addition, the application capability of the distributed circuit in diagnosis of several mechanical failures in a distribution transformer is also investigated.

Analysis of physical transformer circuits for frequency response interpretation and mechanical failure diagnosis

Power transformers are one of the critical points in the renewable energies networks for the transport and distribution of the electric energy. As a result, the diagnosis of transformers is necessary to ensure the reliability and service life. The frequency behavior of the transformer winding is generally studied using a high-frequency model to diagnose the various damages of the winding. For this purpose, this paper discusses the application of the finite element method (FEM) to construct a lumped parameters ladder network of an actual transformer winding. The investigated coil is discretized in five (05) identical sections that includes 30 turns for each. In addition, after the experimental validation of the adopted model, a diagnostic study to the axial deformations of the transformer winding was made using the parameters analysis of the detailed model and the transfer function obtained from the FRA test.

Finite element method to construct a lumped parameter ladder network of the transformer winding

Capacitances in physical distributed circuits of power transformers for simulation based frequency response interpretation have been so far calculated analytically based on internal transformer design and dielectric property data. For the practical application on in-service power transformers, the capacitances should be appropriately determined based on analysis of terminal measurements and simulation. This paper presents a study case on a black-box power transformer for proving this application possibility.

Capacitances in a physical distributed circuit of a black-box power transformer for frequency response analysis at medium frequencies

Abstract The behavior analysis of the transformer is usually achieved by the frequency response analysis (FRA), which is obtained by the application of a very low AC voltage in over a wide frequency range. This paper presents a low and high frequency modelling approach of a three-phase transformer. The developed model consists of a cascade of parallel RLC cells, whose parameters are identified using the frequency response analysis data measurements obtained on each transformer phase. Thus, the proposed model can simulate the frequency behavior of the transformer windings without reference to the geometries of the coils which makes it easily usable in the failure diagnosis field. Experimental results on a 300 VA laboratory transformer validate the proposed model.

Low and high frequency model of three phase transformer by frequency response analysis measurement

In the field of diagnosis of mechanical failures in power transformer’s active part, i.e., windings, leads and the core, the technique of Frequency Response Analysis (FRA) has been recently approved as the main application tool. Mechanical failures in transformer windings reflect changes on measured terminal frequency responses normally in medium frequency range, from several to hundreds of kHz, which is in fact not easy to interpret for diagnosis. The authors proposed a new method based on simulation of a lumped three-phase equivalent circuit of power transformers to interpret frequency responses effectively, but only within low frequency range. This limitation is due to the fact that, the circuit cannot reflect well physical phenomena at medium and higher frequencies. To improve the FRA performance of the proposed method at medium frequencies for transformer failure diagnosis purpose, the paper introduces an investigation on a distributed three-phase equivalent circuit of a 200 kVA 10.4/0.46 kV Yy6 distribution transformer. Result of the investigation is a simplified procedure in determination of electrical parameters associated with the distributed circuit for better simulation based FRA interpretation at medium frequencies.

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Extending analyzed frequency range in interpretation of frequency responses measured on a distribution transformer

The Frequency Response Analysis (FRA) based method is expected as an efficient one in diagnosis of mechanical failures in windings of power transformers, especially in cases other testing methods cannot provide any indication for the diagnostic. However, the current FRA assessment which is based on the relevant Chinese standard and other draft international standards is not sensitive enough in detecting such failures in all cases.

T.M.T. Pham

Papers

Application of a new method in detecting a mechanical failure associated with series capacitance change in a power transformer winding