29-06-2012, 03:15 PM
Modeling Transformers With Internal Incipient Faults
Modeling Transformers With Internal Incipient Faults.doc (Size: 555 KB / Downloads: 47)
INTRODUCTION
INTERNAL winding faults resulting from the degradation of transformer winding insulation can be catastrophic and hence expensive. In the new environment of deregulation, utili- ties therefore are needing inexpensive methods employed to de- tect such faults in the incipient stage. However, the implemen- tations of the existing monitoring methods [1]–[4] tend to cost too much to be applied to distribution transformers. Therefore, an ongoing project in the Power Systems Automation Labora- tory (PSAL) of Texas A&M University is to develop an on-line incipient fault detection method for single-phase distribution transformers that utilizes the terminal parameters of voltages and currents. The development of an accurate internal fault di- agnostic technique for transformers must be based on the anal- ysis of quantities from fault scenarios. Considering the safety of personnel, the damage that will occur in the transformer, the consumed time, and related cost, simulation involving the mod- eling of transformers at various incipient fault stages is the best
way to generate these fault cases.
INTERNAL SHORT CIRCUIT FAULT MODEL
A method was developed to apply finite element analysis to calculate the parameters for an equivalent circuit of the trans- former with an internal short circuit fault using ANSOFT’s Maxwell Software [9]. The 2-D Magnetostatic solver in the package was used to compute the model of the transformer and export an equivalent circuit in the format of SPICE subcircuits. Using finite element analysis to solve problems involves three stages. The first step consists of meshing the problem space into contiguous elements of suitable geometry and assigning appropriate values of the material parameters—conductivity, permeability, and permittivity—to each element. Since an object with permeability equal to 1 in a magnetic model does not need to be modeled, the insulation between the turns and layers were ignored completely.
Incipient Faults With Aging Model Only
Based on the transformer information listed above and the modeling principles discussed in Section II, the parameters of the equivalent circuit for the perfect insulation between two ad- jacent turns of the transformer were calculated. According to the literatures and previous experimental results, the equivalent capacitance, , changes little. Therefore, we fix the equiva- lent capacitance. Then, by changing , the different values of for the various fault scenarios were computed using (3) to represent the insulation in different degraded levels.
CONCLUSIONS
This paper presented a new transformer model to simulate an internal incipient winding fault. The new transformer model was implemented by combining deteriorating insulation model with a finite element analysis internal short circuit fault model. The new deteriorating insulation model, which includes an aging model and an arcing model, was developed based on the phys- ical behavior of aging insulation and the arcing phenomena oc- curring when the insulation was severely damaged. The aging model and the arcing model of the insulation were connected in parallel to produce a parallel combination insulation model. The parallel combination insulation model was combined with the internal short circuit model to predict the terminal voltages and currents of the transformer under various incipient fault condi- tions. The characteristics of the terminal currents and circulating current in the faulted winding were analyzed in time domain and frequency domain.