18-04-2011, 10:58 AM
Submitted By:
Prakash Chandra Sharma
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ABSTRACT
Modelling of transformer with Incipient fault detection in transformers can provide early warning of electrical failure and could prevent catastrophic losses. To develop transformer fault detection technique, a transformer model to simulate internal incipient faults is required. This paper presents a methodology to model internal incipient winding fault 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 behaviour 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 behaviours of the faulted transformer were similar to the simulation results from the incipient faults models.
1. 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 need inexpensive methods employed to detect such faults in the incipient stage. However, the implementations of the existing monitoring methods 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 on-line incipient fault detection methods for single-phase distribution transformers that utilize the terminals 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 modelling of transformers at various incipient fault stages is the best way to generate these fault cases. This paper presents new methodology developed to model internal incipient winding faults in distribution.
Transformers based on the author’s earlier work involving the development of a two-dimensional (2-D) nonlinear finite element analysis internal short circuit model. Since deteriorating insulation involves two stages aging and arcing model and an arcing, the degrading insulation model is composed of an aging model and an arcing model was combined with the internal short circuit model, developed in our earlier work, to simulate internal winding incipient faults. The transformer internal incipient fault model was implemented using commercially available finite element analysis software. Various incipient fault scenarios at different degrading levels of the transformer were analyzed in both time domain and frequency domain. The characteristics obtained from the simulation were compared with the characteristics obtained from some experimental fault cases that conveyed incipient like behaviour.
In this paper, the transformer model to simulate internal short circuit winding faults s briefly introduced in section second. Then the method to simulate an incipient internal winding faults s discussed in more details in section third. In section forth, some simulation results are discussed and compared with experimental results. The conclusions are given in last section.
2. 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. To model an internal fault on the primary or secondary winding, the faulted winding is divided into two subsoil “a” and “b” (turn-to-earth fault) or three subsoil ”a,” ”b,” and ”c” (turn-to-earth fault).
The primary winding and the secondary windings are represented by polygons of corresponding materials. For instance, to simulate a turn-to-earth fault on the primary winding. The transformer is modelled as shown in the fig. 1(a). The primary winding s divided into subsoil a and b. The transformer model to simulate a turn-to-turn fault between two turns on the primary winding is represents as shown in fig. 1(b). The primary winding is divided into three coils a, b and c.
Second, the model has to be excited to set up the initial conditions. Since the sub coil with the maximum number of turns contributes more in the magnetic field, the current source is input into that sub coil with the maximum number of turns contributes more in the magnetic field, the current source is input into that sub coil and all the other coils are open-circuit. In the above example, since sub coil c in fig. 1(b) represents more turns, respectively, they are energized by the current source.
Finally, the boundary conditions for the problems have to be specified. Balloon boundary is a doped in the simulation. Balloon boundaries model the region outside the drawing space as being nearly “infinitely” large; thus, it can effectively isolate the model from other voltage sources.
To simulate an incipient internal winding fault, a model of degraded insulation before break down replaced the switch between two turns to model an incipient winding fault.
3. TLM MDELLING OF TRANSFORMER WITH INTERNAL SHORT CIRCUIT FAULTS
Large power transformers are a class of very expensive and vital components of electric power systems. Since, it is very important to minimize the frequency and duration of unwanted outages, there is a high demand imposed on power transformer protective relays; this includes the requirements of dependability associated with normal-operations, security associated with no false tripping, and operating speed associated with short-fault clearing time. Protection of large power transformers is a very challenging problem in power system relaying (Goshen et al., 2004; Mao and Agawam, 2001).
The development and the validation of algorithms for transformer protection require the preliminary determination of power transformer model. This model must simulate all the situations that will be chosen to chosen to study the behaviour of the protection algorithm. In particular, it must allow for the simulation of internal and external faults. Most of the electromagnetic transient programs available are able to accurately simulate other phenomenon occurring in the transformer like inrush magnetizing current, exciting current and external faults of a power transformer, and applying it to test a transformer protection algorithm, would provide a trust validation ( Lei fried and Fusser, 1999; 1997; Meagher, 2001).
4. 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 cause by the voltage gradient in the insulation. Under normal operating conditions, high voltage gradients below the breakdown voltage do not cause detectable aging. However at elevated temperatures, the structure of a dielectric may be altered significantly during the aging process, and these changes will affect the electrical properties of the dielectric even before insulation failure occurs [13], [14]. The relaxation processes a dielectric undergoes, and hence the characteristics of dielectric loss, depend on the structure. As the structure of the dielectric molecules alters during aging, the dielectric characteristics and electrical properties change. Normal aging of the insulation may be manifested by the gradual reduction in it’s with stand capability over a long period of time. In addition, discharges would like to take place on the surface of the solid insulation or within enclosed voids. The gas ions driven by the electric field would hit on the wall of the insulation and react chemically with some of its surface layer molecules. Thus chemical and thermal degradation of the insulating material occurs at these microscopic sites. When a persistent discharge continues, it is called an arc discharge. This can result in the failure of the electric and a sudden breakdown under operating voltage.
To simulate the incipient internal winding faults completely, both the aging phase and the arcing phase have to be taken into account. Therefore, a combination of an insulation aging model and an arcing model was developed to model an incipient internal winding fault
5. TRANSFORMER INTERNAL INCIPIENT FAULT MODEL
The transformer internal incipient fault computer model is a combination of a two-dimensional nonlinear finite element analysis internal short circuit fault model and deteriorating insulation model consisting of an aging and an arcing component. These models are briefly reviewed in this following section. More details about these models can be found in [2, 4].
5.1 Transformer Non-Linear Model
The transformer two-dimensional nonlinear finite element model presented in [5] applies finite element analysis to calculate the parameters for an equivalent circuit of a transformer with an internal short circuit fault. A soft Maxwell® software package was used to perform calculations. The transformer model can be exported as a SPICE sub-circuit and imported to the SIMPLORER® simulation environment as a black box with a set of interface terminals. These terminals are appropriately connected to the voltage source, load, and incipient fault model to simulate an incipient fault scenario.
5.2 Aging Model
The aging model is traditionally represented by an equivalent parallel RC network as shown in Fig. 1. In the circuit, Rp represents the loss part of the dielectric, which results from electronic and ionic conductivity, dipole orientation and space charge polarization, etc.; and Cp is the capacitance in the presence of the dielectric [6]. In Fig. 1(b) the corresponding phasor diagram of the equivalent circuit is shown. Angle δ is defined as the loss angle, which represents the dielectric energy losses in the insulation. Tan δ is commonly known as the loss tangent or dissipation factor. The loss angle is usually very small for perfect insulation. It differentiates the losses in one dielectric material from those in the other one. Cos θ is the power factor of the dielectric. Equation (1) expresses the relationship between circuit elements and the dissipation factor. When an AC voltage is applied, the capacitive component of the current is IC while the resistive component of the current is IR
