06-03-2013, 11:36 AM
Modeling Transformers With Internal Incipient Faults
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Abstract
Incipient fault detection in transformers can provide
early warning of electrical failure and could prevent catastrophic
losses. To develop transformer incipient fault detection technique,
a transformer model to simulate internal incipient faults is
required. This paper presents a methodology to model internal
incipient winding faults in distribution transformers. These
models were implemented by combining deteriorating insulation
models with an internal short circuit fault model. The internal
short circuit fault model was developed using finite element
analysis. The deteriorating insulation model, including an aging
model and an arcing model connected in parallel, was developed
based on the physical behavior of aging insulation and the arcing
phenomena occurring when the insulation was severely damaged.
The characteristics of the incipient faults from the simulation were
compared with those from some potential experimental incipient
fault cases. The comparison showed the experimentally obtained
characteristics of terminal behaviors of the faulted transformer
were similar to the simulation results from the incipient fault
models.
INTRODUCTION
INTERNAL winding faults resulting from the degradation
of transformer winding insulation can be catastrophic and
hence expensive. In the new environment of deregulation, utilities
therefore are needing inexpensive methods employed to detect
such faults in the incipient stage. However, the implementations
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 Laboratory
(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 diagnostic
technique for transformers must be based on the analysis
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 modeling
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 transformer
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. The core was represented by a
rectangle with two windows. The nonlinear characteristics of
the core were input manually into the solver and assigned to the
core.
Aging Model
In considering the electrical behavior of dielectric material,
it has been traditional to approach the subject in terms of an
equivalent parallel circuit as shown in Fig. 2 [14]. is the applied
voltage and is the current through the insulation. is the
capacitive component of current and the resistive component of
the current is . The resistance represents the lossy part
of the dielectric, taking account of the losses that may result
from electronic and ionic conductivity, dipole orientation and
space charge polarization.
DETERIORATING INSULATION MODEL
The deteriorating insulation between the turns is a major
cause of incipient internal winding faults in transformers.
During the operation of the transformer, a strong electric
field is applied to the dielectric material. It can result in the
aging and deterioration of the insulation. The relevant factors
generally recognized as causing the aging and deterioration
of an insulation include thermal stresses, electrical stresses,
mechanical stresses, moisture, and so on [13]. Thermal stresses
are caused by the internal heating due to current overloads
plus ambient temperatures. Electrical stresses are caused by
the voltage gradient in the insulation. Under normal operating
conditions, high voltage gradients below the breakdown voltage
do not cause detectable aging.
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 adjacent
turns of the transformer were calculated. According to
the literatures and previous experimental results, the equivalent
capacitance, , changes little. Therefore, we fix the equivalent
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. For instance,
to simulate an incipient winding fault between the 15th
turn and the 55th turn on the primary winding, the parameters in
the insulation model for different deteriorated insulation levels
are shown in Table II. The equivalent capacitance for the insulation
between 15th and 55th turns was 89 nF. For the perfect
insulation of Aramid paper as shown in Case 1, the dissipation
factor, , was 0.006. Thus the calculated result for the equivalent
resistance was 49.0 M using (3). With the deterioration
of the insulation, the dissipation factor increased. When the dissipation
factor increased to 3 10 as shown in Case 11, the insulation
was broken down completely. Thus, according to the
similar method, the equivalent resistances for all of the cases
listed in Table II were calculated.
CONCLUSIONS
This paper presented a new transformer model to simulate an
internal incipient winding fault. The newtransformer modelwas
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 physical
behavior of aging insulation and the arcing phenomena occurring
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 conditions.
The characteristics of the terminal currents and circulating
current in the faulted winding were analyzed in time domain and
frequency domain.